1. Field of the Invention
The present invention is directed to an apparatus serving as a control system of a switching control apparatus for an AC circuit, and relates to a voltage control apparatus for controlling power on a power-transmission/distribution line of a power system and power to be supplied to a power-demanding destination. More particularly, this invention relates to a power control apparatus for a power system, such as a power line, which employs a light trigger control system of an AC control circuit using a static induction (SI) thyristor.
2. Description of the Related Art
Conventionally, a triac or a circuit which has thyristors (SCR) or gate turn off thyristors (GTO) coupled in parallel in the opposite bias directions, mainly serves as a power semiconductor device for controlling an AC circuit. There are two typical control systems for an AC circuit using power semiconductor devices. The first system applies an electric trigger signal with a short pulse width simultaneously on two power semiconductor devices coupled in parallel in the opposite bias directions, irrespective of the polarity of a voltage. The second one applies a trigger signal to these devices in accordance with the current phase.
With the use of power semiconductor devices, such as SCR's or GTO's, due to the characteristic of the devices themselves, the switching speed is slow and the ON voltage is high, thus increasing the power loss. Further, the excessive amount of the breakdown voltage with respect to a thermal damage or a sharp increase in voltage, caused by the phenomenon in which a current concentrates within the devices after power has been thrown ON, is smaller as compared with the case involving SI thyristors. Even though the devices are not actually damaged, therefore, the power loss is a bottle neck.
Due to the necessity to perform ON/OFF control of the devices, the gate circuit for SCR's or GTO's is not electrically insulated from the main power circuit, thus raising a problem of a malfunction caused by an electric noise, such as induction noise. Since a pulse transformer or the like is used for ON control according to the prior art, a gate driver inevitably becomes large.
With regard to OFF control, with SCR's in use, natural commutation turn off is applied which turns off the devices when the polarity of an AC current is inverted, while with GTO's in use, a large gate current is required for turning off the devices and the amount of turn-off charges is great, so that the size of the gate circuit becomes large.
In addition, in executing the ON control, it is necessary to match the triggering point with the phase of a current depending on the current phase, so that even in the commutation turn-off system, a current-phase detecting function should be added. Moreover, with a trigger signal having a short pulse width in use, if the triggering point does not coincide with the current phase, the proper ON control may be impossible, nor is the smooth ON control possible. Naturally, the triggering point should precisely coincide with the current phase, thus inevitably making the power controlling apparatus large and complicated.